A DC-DC converter circuit is connected, for example, between first and second DC voltage supplies (hereinafter referred to simply as first and second voltage supplies), and is used as a bidirectional switching circuit that can supply power from a first voltage supply to a second voltage supply, or supply power from a second voltage supply to a first voltage supply, on the basis of the output voltage of the first and second voltage supplies.
For instance, a DC-DC converter circuit is sometimes used in electric vehicles such as work vehicles. An electric vehicle generally has a motor or other such vehicle electric drive unit that is actuated by AC power obtained by converting DC power from an electric accumulation device such as a battery or a capacitor into AC power with an inverter circuit or other such power conversion circuit. The DC-DC converter circuit is provided between the electric accumulation device serving as a first voltage supply and a second voltage supply to which the inverter circuit or other such power conversion circuit is connected, and is designed so that power is supplied from the electric accumulation device to the power conversion circuit in powering mode, whereas power is supplied from the power conversion circuit to the electric accumulation device in regeneration mode.
An example of a conventional DC-DC converter circuit is the chopper circuit discussed in Patent Document 1 (see FIG. 4 in Patent Document 1).
FIG. 19 is a circuit diagram of an example of a conventional DC-DC converter circuit. The DC-DC converter circuit shown in FIG. 19 includes first to fourth switching elements 121 to 124 and an inductor 125.
The first to fourth switching elements 121 to 124 respectively consist of first to fourth semiconductor switches 121a to 124a that allow current to flow in only one direction, and first to fourth diodes 121b to 124b that are respectively connected in parallel to the first to fourth semiconductor switches 121a to 124a so that the first to fourth diodes 121b to 124b allow current to flow in a reverse direction.
The inductor 125 is connected at one end to both the anode side of the first diode 121b included in the first switching element 121 and the cathode side of the second diode 122b included in the second switching element 122, and at the other end to both the anode side of the third diode 123b included in the third switching element 123 and the cathode side of the second diode 124b included in the fourth switching element 124.
With the DC-DC converter circuit shown in FIG. 19, a first voltage supply 110 is connected between the cathode side of the first diode 121b included in the first switching element 121 and the anode side of the second diode 122b included in the second switching element 122, and a second voltage supply 120 is connected between the cathode side of the third diode 123b included in the third switching element 123 and the anode side of the fourth diode 124b included in the fourth switching element 124.
With this conventional DC-DC converter circuit, examples of operating modes indicating the ON and OFF states of the switching elements 121 to 124 include the following first to third modes in which operation is in powering mode, and the following fourth to sixth modes in which operation is in regeneration mode.
FIG. 20 consists of diagrams of the state when the DC-DC converter circuit shown in FIG. 19 is operating in powering mode. Part (a) of FIG. 20 shows the first mode, part (b) of FIG. 20 shows the second mode, and part (c) of FIG. 20 shows the third mode.
For example, in powering mode, as shown in part (a) of FIG. 20, the first mode is one that forms a current path R1a that goes from the first voltage supply 110, through the first switching element 121, the inductor 125, and the fourth switching element 124, and back to the first voltage supply 110. As shown in part (b) of FIG. 20, the second mode is one that forms a current path R2a that goes from the first voltage supply 110, through the first switching element 121, the inductor 125, the third switching element 123, and the second voltage supply 120, and back to the first voltage supply 110. As shown in part (c) of FIG. 20, the third mode is one that forms a current path R3a that goes from the second voltage supply 120, through the second switching element 122, the inductor 125, and the third switching element 123, and back to the second voltage supply 120.
In powering mode, various switching operations are executed such that at least two modes from among the first mode, second mode, and third mode are switched at a short period (such as one selected from a range of about 10 to 100 kHz), according to the magnitude relation between the output voltage V1 of the first voltage supply 110 and the output voltage V2 of the second voltage supply 120.
FIG. 21 consists of diagrams of the state when the DC-DC converter circuit shown in FIG. 19 is operating in regeneration mode. Part (a) of FIG. 21 shows the fourth mode, part (b) of FIG. 21 shows the fifth mode, and part (c) of FIG. 21 shows the sixth mode.
For example, in regeneration mode, as shown in part (a) of FIG. 21, the fourth mode is one that forms a current path R4a that goes from the first voltage supply 110, through the fourth switching element 124, the inductor 125, and the first switching element 121, and back to the first voltage supply 110. As shown in part (b) of FIG. 21, the fifth mode is one that forms a current path R5a that goes from the first voltage supply 110, through the second voltage supply 120, the third switching element 123, the inductor 125, and the first switching element 121, and back to the first voltage supply 110. As shown in part (c) of FIG. 21, the sixth mode is one that forms a current path R6a that goes from the second voltage supply 120, through the third switching element 123, the inductor 125, and the second switching element 122, and back to the second voltage supply 120.
In regeneration mode, various switching operations are executed such that at least two modes from among the fourth mode, fifth mode, and sixth mod are switched at a short period (such as one selected from a range of about 10 to 100 kHz), according to the magnitude relation between the output voltage V1 of the first voltage supply 110 and the output voltage V2 of the second voltage supply 120.